Development characteristics and main controlling factors of the Ordovician carbonate karst in Tahe oilfield

The Middle and Lower Ordovician fractured-vuggy reservoirs in the Tahe oilfield experienced multi-stage karstification caused by multi-stage tectonic events from the Middle Caledonian to the Early Hercynian.Based on integration of geological,geophysical and geochemical data,we discuss the development characteristics and main controlling factors of multi-stage karstification of the Ordovician carbonates in the Tahe oilfield.Four main periods of karstification are identified as follows:the 1st Phase,2nd Phase and 3rd Phase of the Middle Caledonian and the Early Hercynian.The origin,evolution and transformation history of the karst reservoirs are also discussed in this paper.According to this study,the major karstification periods are the Early Hercynian,the 1st Phase and the 2nd Phase of the Middle Caledonian,of which the Early Hercynian is the most important period.The main controlling factors of kastification are paleostructures,karst topography and geological conditions of paleo-drainage-system.